1. Field of the Invention
The present invention relates to an engine unit for a straddle type vehicle.
2. Description of Related Art
Conventionally, there has been known an engine unit including a metal bearing disposed between a rotational shaft fixed to the clutch housing of a centrifugal clutch and rotated therewith and a gear fixed to the clutch boss of the centrifugal clutch and rotated therewith for the purpose of downsizing the unit (for example, see JP-A-2003-301903 and JP-A-2004-251391).
While an engine is idling, the rotational speed of the engine is low, and the centrifugal clutch is OFF (disconnected). Accordingly, the rotational shaft is rotating, but the gear is not rotating. This means that the rotational shaft and the gear are relatively rotating. In contrast, when the vehicle is being driven, the engine speed is high, and the centrifugal clutch is ON (connected). Accordingly, the rotational shaft and the gear are rotating together. This means that the rotational shaft and the gear are not relatively rotating.
As mentioned above, the rotational shaft and the gear relatively rotate only when the engine is idling. During idling operation, the engine speed is kept low. Therefore, it has been believed that abrasions and deterioration of the metal bearing caused by the relative rotation rarely happen.
However, the inventers of the present invention have found through examinations that abrasions and deterioration of metal bearings occur in dependence on conditions of installation of a rotational shaft even when the rotational shaft and a gear do not relatively rotate. More specifically, even if the amount of lubricating oil is increased, abrasions and deterioration of metal bearings occur in a case where the rotational shaft is supported in a cantilever fashion and there is a relatively large difference between respective ratios of lengths of both parts of the rotational shaft which are not supported.
Noticing this, the inventors of the present invention strenuously studied to find the cause of the abrasions and deterioration, and found the cause. The abrasions and deterioration occur because of a mutual prying motion between the rotational shaft and the gear in a structure where the rotational shaft is supported in a cantilever manner even if the rotational shaft and the gear do not relatively rotate. The non-coaxial rotation causes an area where a good oil film is not formed between the outer ring and the inner ring of the metal bearing. In the area, a friction is generated, so that abrasions and deterioration of the metal bearing occur.